An optical disk apparatus (hereinafter called DVD apparatus) that performs reproduction and recording of a DVD (Digital Versatile Disk) is generally provided with an analog front-end LSI which amplifies each of signals outputted from an optical pickup and thereafter takes a difference in signal strength therebetween, thereby obtaining a tracking error signal and a focus error signal of the disk. The DVD apparatus is also provided with a servo control circuit. The servo control circuit drives a motor and an actuator that move the optical pickup and a lens holder based on these error signals, to allow the optical pickup to follow each track of the optical disk and move, thereby performing control for applying a light beam to an accurate position.
In the analog front-end LSI which processes each signal inputted from the optical pickup, a differential amplifier which amplifies a servo system signal and an op amplifier which constitutes an arithmetic unit which performs arithmetic processing between the signals, have offsets due to a variation of the signal of the optical pickup, variations in the manufacture of each circuit, etc., thus causing a case where accurate servo control may not be performed. Since the characteristics differ depending on the type of pickup to be used and an unbalance occurs in each signal due to an assembly error, a balance adjustment circuit or the like for striking a balance between the signals is provided in a signal processor of a servo system. A system for generating the servo system signal needs to perform offset control such that the signal is brought to a desired level. Therefore, the analog front-end LSI is provided with an offset adjustment circuit.
An outline of an offset adjustment system for a servo signal in a conventional analog front-end LSI is shown in FIG. 10. In a DVD apparatus to which the offset adjustment system of FIG. 10 is applied, an arithmetic unit 210 of the analog front-end LSI200 performs arithmetic processing on each signal inputted from an optical pickup 100. A balance adjustment circuit and other circuits 230 for performing other various signal processing perform signal processing such as a balance adjustment between the signals and other various signal processing, and outputs a servo system signal. Thereafter, an AD converter 310 of a digital signal processing LSI300 converts the signal to a digital signal, and an offset amount is calculated by software processing performed by a microcomputer (CPU) or the like operated in accordance with a program. Then, the digital signal processing LSI300 generates a control signal (adjusted value) such that a desired offset is reached, and supplies the same to the analog front-end LSI200, where the control signal is held in a register (or memory) 290, whereby the offset is adjusted. Incidentally, as the invention related to a DVD apparatus in which the signal from the optical pickup 100 is converted into a digital signal after having subjected to arithmetic processing, and an offset adjustment is performed by software processing, there is known the invention described in, for example, the following patent document 1.    Patent Document 1:    Japanese Patent Laid-Open No. 2004-348843